The present invention relates to micro-opto-electro-mechanical systems (MOEMS) and optical modulation. More particularly, the invention relates to the switching and amplitude and phase modulation of signals in optical communications networks such as wavelength-division multiplexing (WDM) architectures.
Over the past 10 years, micro-electro-mechanical systems (MEMS) have emerged as a key enabling technology in several applications. MEMS-based devices used in the automotive industry (accelerometers and pressure sensors) have demonstrated the commercial viability of micro-machining technology, and intense research efforts in fields such as biology, chemistry, and power conversion attest to the exciting possibilities in the new regimes afforded by miniaturization. Optical MEMS, or MOEMS refers to the particular class of MEMS devices that are intended for applications involving the manipulation and use of light. The principal benefits of MEMS devices in the optical domain are parallelism, compactness, low power consumption, and low cost.
Parallelism refers to the ability of simultaneously controlling (modulating) spatially resolved portions of the optical wavefront individually. Compactness is achieved by miniaturizing the mechanical actuators that accomplish light modulation, a possibility that is uniquely offered by MEMS technologies. Low power consumption and compactness go hand-in-hand because miniaturized actuators can be driven with modest power expenditure. Finally, low cost results from the mass-production of micro-fabricated devices using standard semiconductor fabrication materials and manufacturing techniques.
Due to the benefits enumerated above, there is keen interest in the application of MOEMS to the arena of optical switching, in particular in fiber optic networks. The physical modulation of the data in an optical beam can be done in a number of ways. Currently, the dominant method that attracts most interest from researchers and product developers alike is dense wavelength-division multiplexing (DWDM). DWDM is the superposition of several data channels (40xcx9c100) in the same fiber, each channel being carried by a different wavelength. In DWDM, adjacent channels are separated by 0.8 nm in the range of 1530-1565 nm.
In a DWDM network, special devices are needed to modulate the data into different channels, as well as access the modulated data in various conditions, e.g., in order to return specific channels to customers, perform network management functions, etc. A key device for any optical network is a switch. Fundamentally, a switch is a two-port, two-state device: in one state, light is allowed to propagate between one port (the input) and the second port (the output); in the other state, the light path between the two ports is blocked. An Add/Drop switch has more specific functionality in that it manipulates several optical channels in parallel. The xe2x80x9cDropxe2x80x9d operation selects and extracts a single wavelength from the DWDM signal. The xe2x80x9cAddxe2x80x9d operation re-inserts a new data-stream in the DWDM signal, usually at the wavelength extracted by the xe2x80x9cDropxe2x80x9d operation.
Existing commercial Add/Drop switches are fixed, in the sense that the wavelength where data gets dropped or added is set at the time of fabrication. On the other hand, a re-configurable Add/Drop switch would be capable of adding and dropping data at arbitrary wavelengths (within the DWDM range) at any given time; the specific add/drop wavelength would be determined by the operator or by the network management software. The need for re-configurable optical Add-Drop switches is widely acknowledged in the communications industry, because such a device would allow operators of optical fiber networks to reassign leasing and usage attributes with minimum cost and time delay. However, commercial re-configurable Add/Drop switches do not currently exist.
State-of-the art switching devices belong to one of two major categories. One class is called optical-electrical-optical (OEO). In this scheme, the optical signal is transduced into an electrical signal, the signal is switched electrically, and then is reconverted back into a new optical beam. This opto-electrical/electro-optical conversion procedure is limited by the processing speed of the electronics that are available and is costly (each switching station needs demodulators, modulators and electronic equipment).
In the second approach, known as all-optical switching, switching is done entirely in the optical domain. Therefore, many of the problems with the OEO approach are eliminated. Furthermore, this approach is transparent to the protocol being used, the bit rate, or the type of information being sent. All-optical switching can be done with many different technologies, but MOEMS is a major contender for compact, low-cost implementation. Broadly speaking, current MOEMS efforts at optical switches can be characterized either as guided-wave, or as free-space.
Guided-wave switches consist of intersecting rows and columns of waveguides with some sort of switching mechanism at each cross-point. Cross-point switching techniques include pop-up mirrors and thermally actuated bubbles. Typical switching times for these approaches are on the order of 10 milliseconds. Moreover, in each of these approaches the quality of the optical beam is degraded by the switching element. The pop-up mirrors typically have surface quality worse than xcex/5 due to the fabrication process which results in poor reflectivity and high losses. Furthermore, using bubbles as switching elements can introduce lensing effects. Losses in the waveguide material are also unavoidable. Polycrystalline silicon, commonly used as a waveguide material for micro-fabricated devices, can have losses up to 75 dB/cm. While CMOS-quality, single-crystal silicon has an absorption coefficient below 10xe2x88x924 cmxe2x88x921 for 1.3 to 1.55 xcexcm light, formation into waveguide structures using micro-fabrication techniques has yet to be demonstrated. Overall, the losses and optical beam degradation limit the ability to scale up the total port count of these switching elements.
The general architecture of a free-space switch, known as a router, consists of two arrays of mirrors that direct the beam from any channel in an input bank to any channel in the output bank. Clearly, these switches require highly smooth and reflective surfaces to act as mirrors, and in addition, the curvature of the surfaces needs to be carefully controlled. However, these are all highly non-negligible fabrication challenges, and contribute strongly to total insertion loss. Free-space propagation also imposes stringent requirements on the pointing accuracy and precision, and, hence, on the design and performance of the mirror actuators.
The present invention discloses a method for accomplishing general amplitude and phase modulation, and in particular switching in re-configurable Add/Drop switches for optical communications, in a fashion that addresses several of the above-mentioned problems. The principal features of the disclosed approach include low loss of optical power (estimated to be less than 0.1 dB per switched beam), minimal distortion of the switched optical wavefronts, compactness, mass manufacturability (potentially with low costs), and adaptivity to several functionalities.
A micro-opto-electro-mechanical modulator that can amplitude and phase modulate an optical beam. Utilizing frustrated total internal reflection and grating dispersion, the discrete wavelength components of a broadband optical beam can be individually manipulated.
The invention includes an optical substrate and an actuator module. A broadband input optical beam is introduced into the optical substrate such that total internal reflection traps the optical beam in the optical substrate, but allows the beam to leave the optical substrate only at the exit location.
A diffractive element is appropriately located on the optical substrate so as to effect dispersion of the broadband input optical beam into its component wavelengths. This diffractive optical element, which disperses the individual wavelength components to different angles of propagation, is an appropriately located transmission or reflection grating. The input angle and grating periodicity is prescribed such that upon incidence on the top and bottom surfaces of the optical substrate, beams of all input wavelengths are totally internally reflected at the interfaces and remain trapped inside the optical substrate.
The actuator module consists of an array of electrostatically-actuated cantilevers or pneumatically-actuated membranes, hereafter referred to as actuators, and is bonded to the top surface of the optical substrate. The actuators are fabricated on or from, and then recessed from, the top surface of the actuator module. Thus, when the actuator module is subsequently aligned and bonded to the optical substrate, the actuators are free to move in the space between the substrate and actuator surfaces. The alignment is such that each actuator in the actuator array is positioned at the location of a distinct wavelength among those separated from the input beam by the diffractive element.
In an exemplary embodiment, each actuator has two equilibrium positions, one making contact with the optical substrate surface and one remote. Here, contact means that the effective distance between the actuator surface and the optical substrate surface is 1% or less of the optical wavelength, e.g., if the mean wavelength of the input beam is xcex=1.55 xcexcm, contact is effected when the cantilever is within 15 nm from the optical wafer. When the actuator is brought into contact with the optical substrate, the total internal reflection of the corresponding wavelength is completely frustrated, changing the optical path.
By appropriate actuation it is also possible to stabilize the actuators at arbitrary intermediate locations within the free space between the optical substrate and actuator module surfaces. Partial frustration is then accomplished by moving the actuator to intermediate positions between the optical substrate and actuator module surfaces.
An advantage of the invention is that switching and modulation based on frustrated total internal reflection are minimally destructive for the switched beam. In the case where the optical substrate is a silicon wafer, the surfaces are not processed and hence maintain the optical quality of the as-manufactured polished silicon wafer surface. Moreover, the optical path is restricted to silicon alone, which enjoys a low absorption coefficient, as mentioned earlier. These two features ensure maximum efficiency in terms of transmission of optical power and preservation of the integrity of the optical wavefront.
Other advantages of the disclosed invention include: a) robust device architecture and actuation in the sense that the invention is impervious to structural misalignment due to thermal gradients; b) immunity to particulate, fluidic, and other environmental contamination as the cavity where the actuators are moving can be hermetically sealed (potentially at vacuum); c) scalability, reliability, high yield, and ultimately low cost as the fabrication of the device is based on existing micro-machining technology and techniques; and d) low power consumption, since the actuators have low mass and can be designed with high compliance.
A first exemplary embodiment of the invention is a generic amplitude and phase modulator for optical beams. It can be used for conditioning of optical beams, phase correction, and adaptive optics. A second exemplary embodiment is a re-configurable Add/Drop switch for DWDM signals. Design parameters and performance characteristics of these devices are given, and additional technical advantages are readily apparent to those skilled in the art from the figures, descriptions, and claims below.